The invention relates to a method and apparatus for producing an optically effective system of layers on a substrate, such as a lens used in an optical device.
To produce an optical device with defined optical properties, it is known to provide a substrate, in a predetermined way, with a system of layers comprising a plurality of layers with different optical properties, in particular different refractive indices. Depending on the structure of the system of layers, it is possible, for example, to substantially suppress the reflection or transmission in certain wavelength ranges. Systems of layers of this type are used, for example, as an antireflection coating for the lenses of spectacles or for optical filters or mirrors. Standard layer materials are in particular dielectric, such as silicon oxide or silicon nitride. Examples of the structure of antireflection coatings for spectacle lenses are described, for example, in H. Pulker, Optical Coatings on Glass, 2nd edition, Elsevier, Amsterdam 1999. To produce an optical device with a high quality, the individual layers of the layer system must have a predetermined thickness over the entire substrate surface. Furthermore, the substrate must have a predetermined surface structure.
By way of example, sputtering processes are used to produce thin layers, wherein a solid target is bombarded by an ion beam or with ions in a plasma, with the result that individual atoms are removed from the target and deposited on the substrate. To produce optical coatings, a reactive gas, e.g. oxygen or nitrogen, with which the atoms which are being deposited react, is often added to the sputtering gas, e.g. argon. For example, it is known from M. Ruske et al., Properties of SiO2 and Si3N4 layers deposited by MF twin magnetron sputtering using different target materials, Thin Solid Films 351 (1999) 158-163 to produce optical layers of different compositions, e.g. SiO2 and Si3N4, using a single target material by adding, for example, oxygen or nitrogen as reactive gas. In the case of sputtering, it is only possible to coat the side which faces the target. In the case of a substrate which is to be coated on both sides, e.g. a spectacle lens, therefore, the substrate is turned after the front side has been coated, in order for the rear side to be coated.
A problem with plasma-enhanced sputtering processes for producing optical layer systems on a substrate is what is known as the rear side loading, i.e. damage caused, for example, by abrasive material, decomposition, contamination and the like. While one side is being coated, the other side of the substrate undergoes undesired changes on account of the surrounding plasma. In particular, oxygen-containing plasmas damage the substrate surface. Oxygen-containing plasmas are used in all conventional reactive sputtering processes for the deposition of at least one layer material, e.g. of SiO2. The rear side loading is critical for optical elements in which the front and rear sides have to be provided with a well defined system of layers. Also, the surface change can lead to the coating applied failing to bond permanently.
To protect the side which is not being treated from undesirable deposits, it is known to insert the substrate in as tightly fitting a manner as possible into a substrate holder, so that the side which is not being treated does not come into contact with the plasma or the target material. For this purpose, the contour of the substrate holder is matched to the shape of the surface of the substrate in such a way that there is distance of at most 2 mm (dark space distance) between them and it is impossible for any plasma to form in this region. Since the curvatures of ophthalmic lenses vary greatly, a large number of different substrate holders are required and these substrate holders have to be checked and if necessary replaced each time before they are loaded.
It is known from U.S. Pat. No. 6,143,143 to protect that side of an optical lens which is not currently being sputter-coated from undesirable deposits by mechanical means. To do this, it is proposed for the opposite side from the side which is being sputter-coated to be covered either by sticking on a membrane or by the application of a protective gel or spray. A further solution proposed is a substrate holder made from an elastic material, e.g. foam or neoprene, which nestles closely against the substrate. A drawback in this case is the high level of outlay involved in fitting the material and then removing it again in order for the other side to be coated. For this purpose, the sputtering chamber has to be vented during the process. Furthermore, material such as the stuck-on membrane may disrupt the reaction conditions in the sputtering chamber and lead to undesirable deposits or changes in the structure of the layers.
The invention is therefore based on the object of providing a plasma-enhanced sputtering process for the production of an optically effective system of layers on an areal substrate, in which damage to the rear side of the substrate or to the system of layers deposited thereon is substantially avoided. Furthermore, it is intended to provide an apparatus for carrying out the method.